1. Field of the Invention
The present invention relates to a system and method for aligning flexible printing plates on flexographic printing press plate cylinders, and, more particularly, to such a system and method in which aligning pin hole locations are marked on original artwork used for creating the printing plates. The pin hole location marks are then carried through with each step of developing the flexible plates and then, only when the final plates are cut to size and ready to be used on a press, pin register holes are drilled through the finished plate at the location marks. A removable pin register clamp is then attached to the press plate cylinder, the flexible plate with the register holes drilled is placed over the register pins on the clamp, the plate is adhered to the plate cylinder and the clamp is then removed.
2. Description of the Related Art
In flexographic printing presses, flexible printing plates with a raised pattern including a design and/or printed message thereon are adhered to a press plate cylinder, which is positioned adjacent to a backup impression cylinder. Printing ink is applied to the raised design on the plate cylinder and paper to be printed is then pulled between the impression cylinder and the plate cylinder, resulting in an inked imprint of the raised design on the paper.
In order for the press to apply the inked design correctly to the paper, the flexible plate must be precisely aligned on the plate cylinder. This is not a problem on litho presses where the printing plates remain the same size for a given press. The same is true for flexographic presses which use a consistent plate size. In these systems, pin register holes can be installed in the plate material when it is manufactured since the pin register holes can be consistently positioned at the edge of the plate material and the plates are always oriented in a single direction on each plate material sheet.
However, in flexographic presses which use varying sizes of press plates, it is important to keep costs down by maximizing the number of plates which can be made from a single sheet of plate material. It is also important to minimize the amount of plate material which is washed away with solvent after each plate is exposed since it is expensive to dispose of the resulting sludge. Thus, to maximize the use of each plate material sheet and to minimize the amount of solvent used in the plate production process, each sheet is often divided into multiple individual plates which are somewhat randomly positioned on the plate material sheet with their orientations often facing in several different directions. This variety of plate sizes and positions on each sheet means that pin register holes must be located at varying locations on the material sheet. This makes it extremely difficult to place pin register holes at the correct locations in the sheet during the creation and development process. Thus, pin register holes are often dispensed within these composite plates, and scribe lines are used instead. The scribe line method requires a trial and error alignment process, as described below.
In flexographic presses which use different sizes of plates on plate cylinders which are not easily removable, alignment is often accomplished via scribe lines on plate cylinder and plate. The limitations of such a method, including parallax problems and the like, result in what is essentially a trial and error method of plate alignment. For example, in envelope manufacturing plants, where runs of envelopes of varying sizes and shapes must be made on the same presses, it is a complex and time consuming process to accurately position printing plates as they are changed between envelope runs. Often, the changing of a single flexible plate on such a press can take a relatively inexperienced press operator several hours. On a four color press, which employs four different plate cylinders for respectively different ink colors, even an experienced press operator will typically take 3-5 hours to change all of the plates. The trial and error plate changing method involves removing each old plate and adhering a new plate to the plate cylinder via a double sided sticky back tape. Typically, the operator will then run the press to print a number of envelopes. The printed envelopes are carefully examined to determine whether the plate positioning is correct and, if not, the plate position is adjusted accordingly. A press run of envelopes will generally total at least 360 and a typical press can print approximately 800-900 envelopes per minute. Thus, each sampling press run of 30 seconds or so can waste upwards of 400 envelopes. It is not unusual for 10 or more trial and error sampling runs to be made for each plate change. Thus, the costs of changing plates on one press, based upon an optimum press down time of 3 hours and consequent production loss of about 150,000 envelopes, as well as the wastage of 2000-4000 envelopes in the trial and error sampling process, can total several hundred dollars or more.
Prior attempts have been made to provide accurate pin register holes in situations where multiple flexible printing plates are created from a single sheet of plate material. Typically, holes are punched in the material at each stage of the plate production, including hole punching of the board as the artwork is created, repunching the holes in the photographic negative made of the artwork, punching holes in a type flex created from a distortion of the negative, and laying out a plurality of individual type flex plates as a composite on the plate material sheet. The pin register holes are then marked on the sheet, the type flexes removed, holes punched in the sheet, and the type flexes are again laid out on the sheet. With each different set of holes punched, the potential for misalignment increases. Furthermore, the requirement for punching holes at each stage of the plate production process adds significantly to the time and cost of plate production.
It is clear, then, that a need exists for a system and method for accurately and efficiently placing pin registration holes in each plate of a composite sheet of flexographic plates. Once such pin registration holes are positioned, an improved system and method is needed for accurately and consistently positioning the plates on a plate cylinder while the plate cylinder is still on the printing press.